1. Field of the Invention
The present invention generally relates to wireless communications, and more specifically, to a method and system for canceling interference in a wireless receiver.
2. Related Art
An impulse radio system includes an impulse transmitter for transmitting an impulse signal and an impulse receiver spaced from the transmitter for receiving the impulse signal. The impulse signal comprises a train of low power impulses having an ultra-wideband and/or medium wide band frequency characteristic. The impulse receiver samples the low power impulses in the train of impulses to produce a corresponding train of received impulse samples (also referred to as data samples), each having an impulse amplitude. The impulse receiver uses the impulse amplitudes for a variety of purposes, such as for detecting transmitted symbols (that is, for demodulation decisions) and determining separation distances between the impulse radio transmitter and receiver. Therefore, maintaining impulse amplitude accuracy to within a predetermined tolerance correspondingly enhances such processes depending on the impulse amplitudes, including, for example, detecting the presence of impulses and detecting impulse polarity.
Interference can seriously degrade impulse amplitude accuracy. Such interference can include interference having a relatively broadband frequency characteristic, such as random or broadband noise. Also, the interference can have a relatively narrow band frequency characteristic, such as a continuous wave (CW) signal, or a modulated signal, including a frequency, phase, time and amplitude modulated carrier, for example. The impulse receiver is susceptible to both the relatively broadband and the relatively narrow band interference.
When the impulse receiver receives the low power impulses in the presence of relatively narrow band interference, each of the impulse samples (that is, data samples) tends to include both a desired impulse signal component and an undesired interference energy component. Therefore, the relatively narrow band interference can corrupt the impulse amplitudes. Impulse radio randomizing codes can be used to combat the relatively narrow band interference. However, such narrow band interference can often have an amplitude many magnitudes, for example, 20 decibels (dB), larger than an amplitude of the impulse signal. In such instances, the randomizing codes may provide insufficient attenuation of the interference. Additionally, in some instances, randomizing codes are not used in the impulse receiver.
Therefore, there is a need to cancel relatively narrow band interference in an impulse receiver adapted to receive an impulse signal, where the interference can have an amplitude many magnitudes larger than the impulse sample amplitude.
When the impulse receiver receives the low power impulses in the presence of broadband or random noise, each of the impulse samples includes the desired impulse signal component and an undesired random noise component. Since the random noise typically has a low noise power density, it is likely the random noise component and the impulse signal component have comparable amplitudes. Therefore, the random noise component can cause large relative fluctuations in the impulse amplitude, thereby corrupting the impulse amplitude accuracy.
Therefore, there is a need to reduce or eliminate the broadband noise, such as random noise, in an impulse receiver.
There is a further need to cancel the relatively narrow band interference, and at the same time, reduce or eliminate relatively wideband noise in the impulse receiver.
An impulse radio may be frequently used in a mobile environment, for example, as a personal communicator or a locator tag. Therefore it is desirable that such an impulse radio be small and lightweight. These twin goals can be achieved in part by minimizing impulse radio power consumption, and thus battery requirements, and reducing hardware components in the impulse radio.
Therefore, it is desirable to cancel interference in an impulse radio without increasing hardware or power requirements in the impulse radio.
A low duty cycle impulse radio includes an architecture directed to low duty cycle, pulsed operation. Therefore, the low duty cycle impulse radio does not typically include a preponderance of known circuit elements directed to continuous wave transceiver operation, as are found in many types of relatively high duty cycle wireless transceivers, such as in cellular and telephones, Personal Communication Devices (PCS) devices, Pulse Doppler radars, CW ranging equipment, and so on. Such circuit elements can include, for example, phase locked loop (PLL) components such as CW and Voltage Controlled Oscillators, Radio Frequency (RF) and Intermediate Frequency (IF) phase detectors, phase shifters, loop filters and amplifiers. Such relatively high duty cycle transceivers can also include one and two frequency conversion (that is, heterodyning) stages, including frequency mixers and associated IF amplifiers and filters.
It is undesirable to introduce the above mentioned circuit elements into an impulse radio to cancel the relatively high duty cycle interference because of impulse radio cost, size, and power constraints. Moreover, the impulse radio architecture may not be compatible with such circuit elements.
Therefore, there is a need to cancel relatively high duty cycle interference in an impulse radio, using techniques compatible with the low duty cycle architecture of the impulse radio. In other words, there is a need to cancel interference without adding to the impulse radio the exemplary, above mentioned circuit elements more generally associated with high duty cycle transceiver operation.
The present invention has the feature of canceling interference in an impulse receiver adapted to receive an impulse signal, where the interference can have an amplitude many magnitudes greater than an impulse signal amplitude. A related feature of the present invention is to cancel multiple interference signals concurrently received with an impulse signal.
In addition, the present invention has the feature of reducing broadband noise, such as random noise, in an impulse receiver.
The present invention has the advantage of canceling interference in an impulse radio without substantially increasing hardware or power requirements in the impulse radio (for example, without adding analog components dedicated to canceling the interference as is done in conventional interference canceling receivers).
The present invention has the advantage of canceling relatively high duty cycle interference in an impulse radio, using techniques compatible with a low duty cycle architecture of the impulse radio, and thus, without using circuit elements more generally associated with high duty cycle radios.
The present invention relates to methods of canceling interference received by an impulse radio. Additionally the present invention relates to impulse radio receivers that implement the methods of canceling the received interference. In one embodiment, interference canceling involves sampling periodic interference before an expected time of arrival of an impulse in an impulse signal, to produce an interference nulling sample. Then, when the impulse arrives, the impulse is sampled in the presence of the interference to produce a data sample. The anticipatory nulling sample is an estimate of interference energy captured in the subsequent data sample so that the nulling sample can be used to cancel the interference energy from the data sample.
In one embodiment, the nulling sample precedes the data sample by an odd number of half cycle periods of the interference (where a cycle period of the interference=1/(a predictable frequency of the interference), and a half cycle period=the cycle period/2). The nulling sample is additively combined with the data sample to derive a corrected data sample, from which a portion of the interference energy is canceled.
In another embodiment, the nulling sample precedes the data sample by an even number of half cycle periods of the interference. The nulling sample is subtractively combined with the data sample to derive a corrected data sample.
The above described interference canceling technique is based on the frequency of the interference, since the half cycle period is related to the inverse of the frequency. In the present invention the interference frequency is known.